In conventional semi-submersible vessels, a load-supporting, rectangular deck-box structure is positioned upon the top of the columns. Operation modules are then placed on top of the deck-box structure. The deck-box structure offers a structurally solid design and may be of a sealed type, which adds reserve buoyancy to the vessel in an eventual damaged emergency state.
The semi-submersible vessel is used for various services such as production of hydrocarbons, drilling and/or to provide accommodation for personnel. To provide these services, the vessel is equipped with various equipment and systems, which may either be located directly in the deck structure or upon the deck-box structure.
However, a disadvantage with this conventional design is that the operational modules have to be placed relatively high on the vessel which leads to a high center of gravity for the operational modules, and accordingly for the completed vessel. This results in a reduction in stability for the vessel and as a consequence—a lesser pay-load, or alternatively the size of the vessel has to be increased to compensate for the high vertical center of gravity of the operational modules. Furthermore, the weight and the size of these operational modules are normally such that there is only a limited number of devices available that can lift them, a fact that limits the number of available construction sites worldwide.